Engine control systems require accurate control of exhaust gas recirculation (EGR) for controlling regulated emissions and achieving fuel economy improvements. One type of exhaust gas recirculation system externally recirculates the exhaust gas from the exhaust manifold to the intake manifold with a flow control valve placed in the flow path between the exhaust manifold and the intake manifold. Typically, the valve is pneumatically operated and controlled by an electronic engine controller.
One approach to controlling exhaust gas recirculation flow is to use a feedback variable to assure that the actual exhaust gas recirculation flow converges to the desired exhaust gas recirculation flow. One method is to use a differential pressure measured across an orifice in the exhaust flow path upstream of the flow control valve. Then, the differential pressure can be used to infer the actual exhaust gas recirculation flow. The differential pressure measurement provides adequate correlation to exhaust flow because the exhaust pressure varies only slightly in the region where EGR is utilized. Further temperature effects can be accounted for because the upstream exhaust manifold temperature can be correlated to engine operating conditions or ignored due to relatively small variations. Finally, an error between the actual and desired exhaust gas recirculation flow is used to create a control signal that is sent to the flow control valve. Thus, the system can compensate for the effects of engine and component aging, as well as other errors in the system. Such a system is disclosed in U.S. Pat. No. 5,190,017.
The inventors herein have recognized a disadvantage with the above system when the orifice is placed downstream of the valve. In this configuration, flow from the exhaust travels first through a flow control valve and then through the orifice before entering the intake manifold. In this case, the pressure upstream of the orifice (downstream of the valve) varies widely and the assumptions made regarding differential pressure and flow are no longer valid. Also, the temperature upstream of the orifice (downstream of the valve) is no longer correlated directly to engine operating conditions due to the flow expansion in the valve. Thus, there is a significant measurement error when using a differential pressure measurement with a downstream orifice.
One approach to more accurately measure flow is to measure absolute pressure upstream of the orifice, pressure differential across the orifice, and temperature upstream of the orifice. In this way, a correlation between the pressures and temperature can be used to measure the exhaust flow where the pressure and temperatures are widely varying. Alternatively, this approach can be used with the flow control valve where pressure upstream of the valve, pressure differential across the valve, temperature upstream of the valve, and valve area are used to measure flow. Such a system is disclosed in U.S. Pat. No. 4,406,161.
The inventors have recognized a disadvantage with the above approach. The approach requires that upstream temperature be known. Thus, a sensor is needed which adds additional cost and is unacceptable. Further, exhaust manifold temperature estimates based on engine operating conditions inaccurately represent the temperature downstream of a flow control valve. Further, with application of prior art approaches to the flow control valve, valve position, or area, must be measured, adding additional cost.